The present invention relates to printing and in particular to silk screen and other printing processes wherein an apertured screen has selected apertures occluded so that in use, a fluid composition is inhibited from entering them. The occluded apertures are so disposed as to constitute in negative form an image of the design to be printed and the screen is usually a fabric having a relatively open weave to provide the apertures.
In the past such screens have been made by various methods including purely manual techniques whereby the screen has the negative design worked into it by hand-filling the selected apertures with gelatine or shellac. Not only is this a very slow operation requiring great skill, but the product tends to be of limited durability when used on an industrial scale. More commonly, however, photographic methods are used, wherein a photosensitive material is applied to the screen as a sheet or as emulsion. An positive opaque image of the desired print is then superimposed onto the sheet and the whole exposed to light which "fixes" the exposed areas of the photosensitive material outside the opaque image, leaving the part covered by the latter amenable to removal by a solvent treatment.
While these photographic techniques do give improved durability, continued flexing of the screen in use causes progressive damage to the aperture-occluding material which tends to part company with the screen, leaving pinholes and/or jagged edges to the outline of the desired print. There is also a risk that long-term exposure to printing fluids will slowly dissolve out the occluding material. Periodic drying of the screen slows down this leaching process, but it involves stopping production which is often commercially undesirable. Protective coatings can be used, but also have a limited utility, being liable to mechanical damage by the printing process itself.